London Bridge refers to several bridges that have spanned the River Thames between the City of London and Southwark, in central London. The current crossing, which opened to traffic in 1973, is a box girder bridge constructed from concrete and steel. It replaced a 19th-century stone-arched bridge, which in turn superseded a 600-year-old medieval structure. This was preceded by a succession of timber bridges; the first was built by the Roman founders of London.